Method of synthesis of LiTi2(PS4)3

ABSTRACT

A method of synthesis of lithium titanium thiophosphate LiTi 2 (PS 4 ) 3  including the steps of: (a) providing a mixture of lithium sulfide Li 2 S, phosphorus sulfide P 2 S 5  and titanium sulfide TiS 2 ; (b) subjecting the mixture prepared in step (a) to a preliminary reaction step through mechanical milling or melt-quenching to produce an intermediate amorphous sulfide mixture; (c) subjecting the mixture prepared in step (b) to a heat treatment step at a maximum plateau temperature of at least 350° C. and less than 500° C.

FIELD OF THE INVENTION

The present invention relates to a method of synthesis of lithiumtitanium thiophosphate LiTi₂(PS₄)₃.

BACKGROUND ART

The all-solid-state battery system offers the possibility of high energydensity of the battery pack. In order to realize such systems, a solidelectrolyte which exhibits high ionic conductivity is demanded.LiTi₂(PS₄)₃ is a candidate for such a solid electrolyte, and has beendescribed in Kim et al., Chem. Mater. 2008, 20, 470-474; Kim et al.,Electrochemistry Communications 10 (2008) 497-501; and Shin et al.,Journal of The Electrochemical Society, 161 (1) A154-A159 (2014).

According to the method of synthesis described in Kim et al., Chem.Mater. 2008, 20, 470-474, a stoichiometric mixture of Li₂S, TiS₂ andP₂S₅ is mixed and heated under vacuum according to the followingtemperature profile:

However, the synthesis conditions proposed in the literature were notoptimized with respect to the generation of secondary phases and ionicconductivity.

SUMMARY OF THE INVENTION

With a view to solving the above-referenced problems in the synthesis oflithium titanium thiophosphate LiTi₂(PS₄)₃, the present inventor hasstudied different aspects of the preparation of this material, and thiswork has led to the achievement of the present invention.

In one aspect, the present invention thus relates to a method ofsynthesis of lithium titanium thiophosphate LiTi₂(PS₄)₃ comprising thesteps of:

(a) providing a mixture of lithium sulfide Li₂S, phosphorus sulfide P₂S₅and titanium sulfide TiS₂;

(b) subjecting the mixture prepared in step (a) to a preliminaryreaction step through mechanical milling or melt-quenching to produce anintermediate amorphous sulfide mixture;

(c) subjecting the mixture prepared in step (b) to a heat treatment stepat a maximum plateau temperature of at least 350° C. and less than 500°C.

Here “maximum plateau temperature” refers to the maximum temperaturemaintained in a heating vessel as commonly used in solid statechemistry, the temperature vs. time profile typically including agradual ascent phase with a controlled rate of increase of temperaturestarting from room temperature, a chosen reaction temperature maintainedeffectively constant over a period of time (the “maximum plateautemperature”), and then a descent phase wherein the temperature isbrought back down to room temperature.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1a shows a schematic view of a high temperature heat treatmentprocess according to a prior art method, with a temperature inclineprior to heat treatment, a maximum plateau temperature maintained at700° C. during heat treatment and then a programmed temperaturedecrease. FIG. 1b shows an illustrative and non-limiting embodiment of aprocess according to the present invention with a first amorphasizingstep, followed by low-temperature heat treatment, with a preferredmaximum plateau temperature T maintained during heat treatment at 375°C.≤T≤450° C.

FIG. 2 shows X-ray diffraction patterns (XRD) of lithium titaniumthiophosphate LiTi₂(PS₄)₃ samples obtained according to the process ofthe present invention (embodiments illustrated in a non-limiting mannerby Examples 1 to 3) or according to the conventional synthesis process(Comparative Example 1) or according to other processes not according tothe present invention (Comparative Examples 2 to 4).

FIG. 3 shows thermogravimetric analysis (TGA) study of high temperatureheat treatment of amorphasized LiTi₂(PS₄)₃ (Comparative Example 4).

FIG. 4 shows ionic conductivity values observed for lithium titaniumthiophosphate LiTi₂(PS₄)₃ samples obtained according to illustrative andnon-limiting embodiments of the present invention (Examples 1 to 3), oraccording to embodiments not according to the present invention(Comparative Examples 3 and 4).

DETAILED DESCRIPTION OF THE INVENTION

Without wishing to be bound by any particular theory, it is believedthat the low ionic conductivity of LiTi₂(PS₄)₃ obtained according toliterature methods may be due to the impurity of TiS₂ included in thesynthesized material. Because of the incongruent melting behavior ofLiTi₂(PS₄)₃, it is important to avoid melting of the material duringheat treatment. By “incongruent melting”, reference is made here tochemical transformation of the material upon melting, with new chemicalbonds being formed. Without control of undesired melting and impurityformation, it is very difficult to obtain a single phase of LiTi₂(PS₄)₃.To solve the above-mentioned problems, the present invention provides anew synthesis procedure of LiTi₂(PS₄)₃.

In the present invention, an intermediate step is proposed, before thehigh-temperature reaction to produce final LiTi₂(PS₄)₃ product, theintermediate step consisting of subjecting the mixture prepared in step(a), based on ordinary solid samples of each of the three reactioncomponents (as may be obtained from commercial suppliers) to apreliminary reaction step through mechanical milling or melt-quenchingto produce an intermediate amorphous sulfide mixture. This intermediatestep may also be referred to hereinafter as an “amorphasizing step”, andprovides amorphous materials derived from Li₂S—TiS₂—P₂S₅. This amorphousmaterial is able to be heat-treated at over 400° C. without melting.Without the intermediate step, the specimens are melted due to the lowmelting point of P₂S₅ (˜270° C.). Elemental phosphorus and sulfur alsohave lower melting points and will thus melt before the temperaturesneeded to produce LiTi₂(PS₄)₃ product in the solid phase reaction. It isconsidered that the most important effect of the intermediate step isallowing Li₂S, TiS₂ and P₂S₅ to completely mix and react with eachother—the intermediate step may, as well as ensuring intimate physicalmixing, begin the chemical transformations which will lead to the finalproduct. The intermediate step may thus act to react the low meltingmaterial of P₂S₅ with Li₂S (or at least start this reaction).

In terms of starting material mole ratios, the method is appropriatelycarried out with as close as possible to a stoichiometric ratio thereofin view of the final product LiTi₂(PS₄)₃ product to be produced. Thus,the lithium sulfide Li₂S, phosphorus sulfide P₂S₅ and titanium sulfideTiS₂ starting materials are generally used in a Li₂S:P₂S₅:TiS₂ moleratio of 1:3:4.

The amorphous material obtained through the “amorphasizing step” canadvantageously be heat-treated at the temperature of 350° C.≤T<500° C.More generally, in heat treatment step (c), the maximum plateautemperature is appropriately not more than 475° C., and preferably atleast 375° C., more preferably at least 400° C. and at most 450° C.Further, in heat treatment step (c), the maximum plateau temperatureduring heat treatment is appropriately maintained for at least 1 hourand at most 300 hours. In terms of the speed of temperature increasegoing from room temperature up to the maximum plateau temperature,before the heat treatment step, a generally appropriate range is from0.1° C. min⁻¹ to 20° C. min⁻¹. A preferred speed is in the range of 1°C. min⁻¹ to 5° C. min⁻¹. Analogous rates of temperature decrease afterthe heat treatment step, may also be used to bring the sample back downto room temperature.

When LiTi₂(PS₄)₃ is produced by using the method of the presentinvention, it can be obtained substantially as a single phase withoutany impurity of TiS₂. Values of about three times higher ionicconductivity can be observed as compared to that of LiTi₂(PS₄)₃ producedby a conventional process.

In the method according to the present invention, each of the methodsteps (a), (b) and (c) is advantageously carried out under an inert gas,for example, nitrogen or argon, preferably argon.

As mentioned above, in one way of carrying out the present invention,the intermediate step (b), also referred to herein as an “amorphasizingstep”, the intermediate step giving rise to an intermediate amorphoussulfide mixture, may be a carried out by a “melt-quenching” procedure.In an appropriate melt-quenching step, the starting materials are heatedto a temperature higher than the melting point of the final productLiTi₂(PS₄)₃, i.e. to over 700° C. A quick heating procedure may be usedto avoid evaporation when attaining the high temperature of over 700° C.However, in most cases, it is preferable for a sample to be in theequilibrium state before quenching. Therefore, it is appropriate to waitfor a relatively longer period and heat up more slowly, for example witha heating rate of 0.05° C. min⁻¹ to 20° C. min⁻¹, with a holding time ofappropriately 3 hours to 300 hours. To quench the molten mixture, raisedto a temperature above 700° C., a rapid cooling rate is used,appropriately between 300 to 1,000 K s⁻¹, to bring the mixture to roomtemperature or below. A generally appropriate method for sulfideamorphous materials is ice quenching. In such a method, a heated quartztube containing the material to be quenched is placed in ice water.

In preferred embodiments for carrying out the present invention, theintermediate step (b), also referred to herein as an “amorphasizingstep”, the intermediate step giving rise to an intermediate amorphoussulfide mixture, is carried out by a mechanical milling procedure, suchas planetary ball milling, vibration milling or jet milling. Where thepreferred method of planetary ball milling is used, a generallyappropriate ball size range is chosen within the range 1 mm≤ϕ≤10 mm, thetemperature range is chosen within the range 0° C.≤T≤60° C., therotation speed is chosen within the range 200 rpm≤R≤500 rpm, and theduration is chosen within the range 5≤t≤200 h.

In a further aspect, the present disclosure relates to anall-solid-state lithium battery comprising the following elements:

-   -   a positive electrode active material layer;    -   a solid electrolyte layer;    -   a negative electrode active material layer,        wherein the solid electrolyte layer contains a lithium titanium        thiophosphate LiTi₂(PS₄)₃ material produced according to the        present invention, and is positioned between the positive        electrode active material layer and negative electrode active        material layer.

In such an all-solid-state lithium battery, using as solid electrolyte,the LiTi₂(PS₄)₃ sulfide materials produced according to the presentinvention, concerning the form of the solid electrolyte materials,examples include a particle shape, such as the shape of a true ball andthe shape of an elliptical ball, or a thin film form, for example. Whensolid electrolyte materials have a particle shape, as for the meanparticle diameter, it is preferable that their size is within the rangeof 50 nm to 10 micrometers, more preferably within the range of 100 nmto 5 micrometers.

Although it is preferable to have only one or more solid electrolytematerials as mentioned above in a solid electrolyte layer, this layermay also contain a binding agent if needed. As a binding agent used fora solid electrolyte layer, this may be of the same type as mentionedhereunder for the positive active material layer.

As regards the thickness of a solid electrolyte layer, although this maychange with the kind of solid electrolyte materials, and the overallcomposition of an all-solid battery, generally it is preferable thatthis thickness is within the range of 0.1 micrometer to 1,000micrometers, more preferably within the range of 0.1 micrometer to 300micrometers.

Concerning the positive active material (cathode active material) to beused in the positive electrode (cathode) active material layer, this isnot especially limited if the average operating potential becomes morethan 4 V (vs. Li/Li⁺). As an average operating potential of positiveactive material, this is appropriately more than 4 V (vs. Li/Li⁺), andit is preferable that it is within the limits of 4.0 V to 6.0 V, stillmore preferably within the limits of 4.5 V to 5.5 V. The averageoperating potential can be evaluated using cyclic voltammetry, forexample. In particular, when cyclic voltammetry is measured at a smallelectric potential speed like 0.1 mV/sec, it can be considered that theaverage value of the voltage which gives the peak current on the side ofoxidation, and the voltage which gives the peak current on the side ofreduction is the average operating potential.

As a positive active material, especially if the average operatingpotential is made with more than 4 V (vs. Li/Li⁺), there is no specificlimitation, but it is preferable that the material is an oxide positiveactive material, which can have a high energy density.

A compound which has the spinel type structure denoted by generalformula LiM₂O₄ (M is at least one kind of transition metal element), asan example of positive active material, can be mentioned as an example.As regards M of the above-mentioned general formula LiM₂O₄, especiallyif it is a transition metal element, it will not be limited, but it ispreferable that it is at least one kind chosen from the group whichconsists of Ni, Mn, Cr, Co, V, and Ti, for example, and it is morepreferable that it is at least one kind chosen from the group whichconsists of Ni, Mn, and Cr especially. Specifically,LiCr_(0.05)Ni_(0.50)Mn_(1.45)O₄, LiCrMnO₄, LiNi_(0.5)Mn_(1.5)O₄, etc.can be mentioned. The compound which has the olivine type structuredenoted by general formula LiMPO₄ (M is at least one kind of transitionmetal element) as other examples of positive active material can bementioned. M in the above-mentioned general formula will not be limitedespecially if it is a transition metal element, but it is preferablethat it is at least one kind chosen from Mn, Co, Ni, and the group thatconsists of V, for example, and it is more preferable that it is atleast one kind chosen from the group which consists of Mn, Co, and Niespecially. Specifically, LiMnPO₄, LiCoPO₄, LiNiPO₄, etc. can bementioned. The compound which has the layer structure denoted by generalformula LiMO₂ (M is at least 1 type of a transition metal element) asother examples of positive active material can be mentioned.Specifically, LiCoO₂, LiNi_(0.5)Mn_(0.5)O₂ andLiNi_(0.33)Co_(0.33)Mn_(0.33)O₂ etc. can be mentioned. As examples otherthan the positive active material mentioned above, aLi₂MnO₃—LiNi_(1/3)Co_(1/3)Mn_(1/3)O₂ solid solution, aLi₂MnO₃—LiNi_(0.5)Mn_(1.5)O₂ solid solution, a Li₂MnO₃—LiFeO₂ solidsolution, etc. can be mentioned.

As regards the form of the positive active material, a particle shape,such as the shape of a true ball and the shape of an elliptical ball,thin film form, etc. can be mentioned, as an example. As for the meanparticle diameter, when the positive active material has a particleshape, it is preferable that it is within the size range of 0.1micrometer to 50 micrometers, for example. As for the content of thepositive active material in a positive active material layer, it ispreferable that it is in the range of 10% by weight to 99% by weight,for example, more preferably from 20% by weight to 90% by weight.

Concerning the positive active material layer, in addition to thepositive active material mentioned above, if needed, the positive activematerial layer in may contain other materials, for example, solidelectrolyte materials etc. As for the content of the solid electrolytematerials in a positive active material layer, it is preferable thatthis content is 1% by weight to 90% by weight, more preferably 10% byweight to 80% by weight.

Furthermore, a positive active material layer may contain anelectrically conductive agent from a viewpoint of improving theconductivity of a positive active material layer, other than the solidelectrolyte materials mentioned above. As electrically conductivematerial, acetylene black, Ketjenblack, a carbon fiber, etc. can bementioned, for example. A positive active material may also contain abinding agent. As such a binding material (binding agent),fluorine-based binding materials, such as polyvinylidene fluoride (PVDF)and polytetrafluoroethylene (PTFE), etc. can be mentioned, for example.

Although the thickness of a positive active material layer may changeaccording to the kind of all-solid-state battery made, it is generallypreferable that it is within the range of 0.1 micrometer to 1,000micrometers.

As regards the negative electrode active material layer, this layer atleast contains one or more negative electrode active material(s), andmay additionally contain at least one or more of solid electrolytematerials and electrically conductive agents if needed. Forall-solid-state lithium batteries, the negative electrode activematerial is not limited provided that occlusion and discharge of the Liion, which is a conduction ion, are possible. As a negative electrodeactive material, a carbon active material, a metal active material, etc.can be mentioned, for example. As a carbon active material, black lead,meso carbon micro beads (MCMB), highly ordered/oriented pyrolyticgraphite (HOPG), hard carbon, soft carbon, etc. can be mentioned asexamples. On the other hand, as a metal active material, charges of analloy, such as Li alloy and Sn—Co—C, In, Al, Si, Sn, etc. can bementioned as examples. Oxide stock materials, such as Li₄Ti₅O₁₂, can bementioned as examples of other negative electrode active materials.

Concerning solid electrolyte materials used for the negative electrodeactive material layer, and an electrically conductive agent, these maybe the same as that for the solid electrolyte layer and positive activematerial layer mentioned above.

The thickness of the negative electrode active material layer willgenerally be appropriately within the range of 0.1 micrometer to 1,000micrometers.

An all-solid-state battery of the present disclosure has at least thepositive active material layer, solid electrolyte layer, and negativeelectrode active material layer which were mentioned above. It furtherusually has a positive pole collector which collects a positive activematerial layer, and a negative pole collector which performs currentcollection of a negative electrode active material layer. As a materialof a positive pole collector, for example, SUS (stainless steel),aluminum, nickel, iron, titanium, carbon, etc. can be mentioned, and SUSis especially preferable. On the other hand as a material of a negativepole collector, SUS, copper, nickel, carbon, etc. can be mentioned, forexample, and SUS is especially preferable. Concerning the thickness,form, etc. of a positive pole collector and a negative pole collector,the person skilled in the art may choose suitably according to the useof the all-solid-state battery, etc. The cell case used for a commonall-solid-state battery can be used, for example, the cell case madefrom SUS, etc. can be mentioned. The all-solid-state battery may form apower generation element in the inside of an insulating ring.

The all-solid-state battery of the present disclosure can be consideredas a chargeable and dischargeable all-solid-state battery in a roomtemperature environment. Although it may be a primary battery and may bea rechargeable battery, it is especially preferable that it is arechargeable battery. Concerning the form of the all-solid-statebattery, a coin type, a laminated type, cylindrical, a square shape,etc. can be mentioned, as examples.

As regards the manufacturing method of the all-solid-state battery, thisis not particularly limited, and common manufacturing methods ofall-solid-state batteries can be used. For example, when anall-solid-state battery is in the thin film form, a positive activematerial layer can be formed on a substrate, and the method of forming asolid electrolyte layer and a negative electrode active material layerin order, and laminating them thereafter etc., may be used.

Within the practice of the present invention, it may be envisaged tocombine any features or embodiments which have hereinabove beenseparately set out and indicated to be advantageous, preferable,appropriate or otherwise generally applicable in the practice of theinvention. The present description should be considered to include allsuch combinations of features or embodiments described herein unlesssuch combinations are said herein to be mutually exclusive or areclearly understood in context to be mutually exclusive.

EXPERIMENTAL SECTION—EXAMPLES

The following experimental section illustrates experimentally thepractice of the present invention, but the scope of the invention is notto be considered to be limited to the specific examples that follow.

Example 1

Synthesis of Solid Electrolytes

Mixture Step

The solid electrolyte LiTi₂(PS₄)₃ was synthesized using startingmaterials Li₂S (Sigma), TiS₂ (Sigma), and P₂S₅ (Aldrich). They weremixed at the weight ratio listed in Table 1 below:

TABLE 1 Weight ratio of starting materials for LiTi₂(PS₄)₃ synthesisWeight of starting material (g) mmol of starting material Li₂S 0.0395840.8615 P₂S₅ 0.57447 2.5845 TiS₂ 0.385946 3.4460

Amorphasizing Step

The mixed sample was put into a zirconium pot (45 mL) with 18 zirconiumballs (ϕ 10 mm) under argon. The pot was closed and treated withplanetary milling equipment (Fritsch, P7) at 370 rpm for 40 h to obtainthe precursor.

Heat Treatment Step

The precursor was sealed into a glass tube at a pressure of 30 Pa andthen heated at T=375° C. for 8 h. A temperature incline of 2.2° C. min⁻¹was used to bring the sample up to this heat treatment temperature.

At the heat treatment temperature of 375° C., the sample was not melted.

The procedure is shown in FIG. 1 as well as the literature process.

Measurement of Li Ion Conductance

Li ion conductance at a temperature of 25° C. was measured using thesulfide solid electrolyte material obtained. First, 100 mg of thesulfide solid electrolyte material was added to a cylinder made ofalumina and pressed at 4 ton/cm² to form a solid electrolyte layer. Thepellet was sandwiched by SUS current collector for measuring impedancespectroscopy.

An impedance gain-phase analyzer manufactured by Biologic (VMP3) wasused for the measurement as FRA (Frequency Response Analyzer). Themeasurement was started from a high-frequency range with the conditionsof an alternating voltage of 5 mV and a frequency range of 1 Hz to 1MHz.

The ionic conductivity of the material prepared in Example 1 was2.1×10⁻⁵ S/cm.

X-Ray Diffraction Measurement

X-ray diffraction measurement (using a CuKα line) was performed by usingthe sulfide solid electrolyte materials obtained in Examples and inComparative Examples. The results are shown in FIG. 2.

For samples prepared according to the Examples, essentially only thepeaks of LiTi₂(PS₄)₃ were detected.

Example 2

Only the heat treatment temperature was different from Example 1—herethe heat treatment temperature was 400° C. and the sample was notmelted. The ionic conductivity of the material produced by Example 2 was2.5×10⁻⁵ S/cm. Essentially only the peaks of LiTi₂(PS₄)₃ were detectedby XRD.

Example 3

Only the heat treatment temperature was different from Example 1—herethe heat treatment temperature was 450° C. and the sample was notmelted. The ionic conductivity of the material produced by Example 3 was2.0×10⁻⁵ S/cm. Essentially only the peaks of LiTi₂(PS₄)₃ were detectedby XRD.

Comparative Example 1=Synthesized Via Literature Process

Unlike Example 1, no amorphasizing step was performed. The heattreatment temperature was 750° C. and the sample was melted. The ionicconductivity of this sample was 0.9×10⁻⁵ S/cm. Peaks of TiS₂ weredetected by XRD in addition to those attributed to LiTi₂(PS₄)₃.

Comparative Example 2

As a difference from Example 1, no amorphasizing step was performed. Theheat treatment temperature was 750° C. and the sample was melted. Theionic conductivity of this sample was 0.6×10⁻⁵ S/cm. Peaks of unknownphase were detected by XRD.

Comparative Example 3

As a difference from Example 1, the amorphasizing step was performed buta heat treatment temperature of 300° C. was used, and the sample was notmelted. The ionic conductivity of this sample was 0.2×10⁻⁵ S/cm. A halopattern was detected by XRD, indicating an amorphous product.

Comparative Example 4

As a difference from Example 1, the amorphasizing step was performed buta heat treatment temperature of 500° C. was used, and the sample was notmelted. The ionic conductivity of this sample was 1.2×10⁻⁵ S/cm. Peaksof TiS₂ were detected in addition to LiTi₂(PS₄)₃ by XRD.

From the TGA measurement, weight loss was observed at around 400° C.(FIG. 3). This indicates evaporation of the material. Such anevaporation may lead to the decomposition of LiTi₂(PS₄)₃ and thegeneration of TiS₂ at 500° C. Here, it is considered that this is notunreacted titanium disulfide but instead (re)generated TiS₂, issuingfrom LiTi₂(PS₄)₃ already produced. It is observed that once the TiS₂ is(re)generated during heat treatment, it is very difficult to remove itby heat treatment (a very long time is needed)

Importance of the Amorphasizing Step

From the results Comparative Example 1 and Comparative Example 2 withoutan amorphasizing step, it is very difficult to avoid the melting of thematerial due to the low melting point of P₂S₅ (˜270° C.). LiTi₂(PS₄)₃was obtained at a high temperature heat treatment condition of T=750° C.However, the material obtained included TiS₂ as impurity. Possibly as aresult of this, its ionic conductivity was low. Even though the lowtemperature of T=400° C. (same as Example 1) was applied, the materialwas melted, resulting in an unknown phase and low ionic conductivity.The amorphasizing step allows one to apply a temperature of over 400° C.without melting. Therefore it is possible to synthesize a single phaseof LiTi₂(PS₄)₃ exhibiting high ionic conductivity.

Temperature Optimization

From the results of Example 1&2 and Comparative Examples 3&4, the heattreatment temperature T range of 375° C.≤T<500° C. is preferable toobtain the single phase of LiTi₂(PS₄)₃ and high ionic conductivity (FIG.4). A temperature of 300° C. or lower gives rise to amorphous productand a temperature of 500° C. or higher appears to be correlated with thecontinued presence of TiS₂ and evaporation.

The invention claimed is:
 1. A method of synthesis of lithium titaniumthiophosphate LiTi₂(PS₄)₃ comprising the steps of: (a) providing amixture including lithium sulfide Li₂S, phosphorus sulfide P₂S₅ andtitanium sulfide TiS₂; (b) subjecting the mixture prepared in step (a)to a preliminary reaction step through melt-quenching to produce anintermediate amorphous sulfide mixture, wherein the melt-quenchingincludes heating the mixture prepared in step (a) at a heating rate thatis in a range of from 0.05° C./min to 20° C./min, and for a holding timethat is in a range of from 3 hours to 300 hours; (c) subjecting theintermediate amorphous sulfide mixture prepared in step (b) to a heattreatment step at a maximum plateau temperature that is above 400° C.and less than 500° C.
 2. The method according to claim 1, wherein in theheat treatment step (c), the maximum plateau temperature is not morethan 475° C.
 3. The method according to claim 1, wherein in the heattreatment step (c), the maximum plateau temperature during the heattreatment is maintained for at least 1 hour and at most 300 hours. 4.The method according to claim 1, wherein the lithium sulfide Li₂S,phosphorus sulfide P₂S₅ and titanium sulfide TiS₂ starting materials areused in an Li₂S:P₂S₅:TiS₂ mole ratio of 1:3:4.
 5. The method accordingto claim 1, wherein the melt-quenching further includes heating themixture prepared in step (a) to a temperature above 700° C. to melt themixture, and cooling the heated mixture at a cooling rate of between 300to 1,000 K/s.
 6. The method according to claim 1, wherein the maximumplateau temperature is in a range of from 450° C. to less than 500° C.